Image 1
	Meltwater stream flowing into a large moulin in the ablation zone (area below the equilibrium line) of the Greenland ice sheet. Photo Courtesy: Roger J. Braithwaite, The University of Manchester, UK

New measurements show that the flow of ice in the Greenland ice sheet has been accelerating since 1996 during the summer melt season. The results suggest that the ice sheet may be responding more quickly to the warming climate than previously thought.

In an article published in Science magazine's online Sciencexpress June 7, Jay Zwally, an ICESat Project scientist at the NASA Goddard Space Flight Center, Greenbelt, Md., Waleed Abdalati, Polar Program scientist at NASA Headquarters, Washington, and colleagues report that increases in ice velocity during the summer are correlated with the timing and the intensity of ice sheet surface melting.

Using periodic Global Positioning Satellite measurements from 1996 through 1999, the researchers discovered that the ice flow speeds up from 31.3 cm (12.3 inches) per day in winter to a peak of 40 cm (15.7 inches) per day in the summer when surface melting is largest. "This study demonstrates that surface meltwater travels quickly through the 1200 meter (approx. 3/4 mile) thick ice to the bedrock to make the ice slide faster. This process was known for decades to enhance the flow of small mountain glaciers, but was not known to occur in the large ice sheets," Zwally said.

	Image 2
	This schematic highlights glaciological features of the ice sheet including surface lakes, crevasses, and large openings called moulins, that stretch up to 10 meters in diameter and drain to the bedrock. Meltwater descends through the moulins, down to the bedrock, contributing to the movement of the ice sheet. The Equilibrium Line (EQ LINE), similar to a snow line on the glacier, is at about 1200 meters elevation in west-central Greenland. In the ablation zone below the EQ LINE, all the winter snow plus some of the ice flowing from higher elevations melts each summer. "GPS" is the marker where the "Global PositioningSystem" was located to observe the movement of the ice sheet. The "V" indicates the velocity of the movement of the ice, some of which is from sliding over the bedrock.

The meltwater makes its way from the surface to the bedrock by draining into crevasses and large tunnels called moulins that may be as large as 10 meters (approx. 33 feet) in diameter. More meltwater underneath the ice sheet provides lubrication to allow the ice sheet to move faster toward the coastline of Greenland.

Over time, as ice melts from the top of the ice sheet, the ice thins and spreads out toward lower elevations closer to the coast. The meltwater also carries heat (in the form of water) from the top of the ice to the base of the ice that sits on the bedrock.

	Image 3
	These are elevation contours (50 m or 164 feet) overlain on a Landsat image taken on June 22, 1990, which is typically about 1/3 of the way through the melt season. The greyer areas at lower elevations in the image are bare ice, with some whiter patches of remaining winter snow near the ice-snow line. By the end of the melt season in late August to early September the firn-ice (Firn is snow that is more than one year old) boundary usually retreats to around the average location of the equilibrium line near the Swiss Camp. The dark patches are melt lakes, some of which show dark lines of inflow channels. Later in the season, melt lakes also form above the equilibrium line.

A separate study by Abdalati and Konrad Steffen of the University of Colorado showed that the melting of the ice sheet surface has been increasing at a rate of nearly 20% over the last 21 years, while summer temperatures in that same period have increased by one quarter of a degree Celsius (.45º Fahrenheit). The link between ice sheet melt and ice flow suggest that the increasing melt may be more significant than previously believed.

The faster ice flow, ice thinning and consequent lowering of the surface elevation of the ice sheet can open a feedback to more melting that has not been considered in computer models that predict ice sheet response to climate change.

NASA's ICESat mission, which is planned for launch in December of this year, will use a laser altimeter to monitor ice sheet elevations and show elevation changes as small as 1 cm/yr. ICESat's measurements will tell us whether the ice sheets are growing or shrinking overall, how much they are contributing to sea level change, and will help predict future changes in ice volume and sea level.

Zwally and his colleagues speculate that increased movement of the ice sheet due to more meltwater underneath the ice sheet may have contributed to the demise of the Laurentide ice sheet approximately 10,000 years ago. During that time, the Earth's axis of rotation was more tilted toward the Sun causing warmer summers in the Northern Hemisphere.

Further, they suggest that the observed process may also have contributed to the extensive melting of the Greenland Ice Sheet during the last Interglacial period, some 125,000 years ago. According to the U.S. Geological Survey, previous studies have shown that during the last Interglacial period, carbon dioxide (CO2) concentrations in the atmosphere were relatively high, temperatures may have been higher than the present, and sea level may have been approximately 6 meters (19.5 feet) higher.

"During this time when the climate was warmer, the ice sheet was less extensive. With the predicted greenhouse warming we may be returning to similar conditions," Zwally said.

This research was funded under NASA's ICESat Project.

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